Ultrasound is an increasingly used tool for noninvasively examining a patient's body. A typical ultrasound system works by transmitting high frequency acoustic signals into the body and detecting and analyzing the returned echoes. To create an image of the tissue in the body, the strength of the ultrasound echo is determined and used to modify the intensity of pixels in a digital display screen.
For reasons set forth below, the resolution with which the internal body matter can be imaged is highly dependent upon the speed at which the ultrasonic pulses travel in the body. The speed of sound is known to vary with tissue type. For example, the speed of sound in fat tissue varies between 1410 and 1479 m/sec., while in liver tissue the speed of sound varies between 1553 and 1607 m/sec. For kidney tissue, the speed of sound varies between 1558 and 1568 m/sec., while in muscle tissue, the speed of sound varies between 1543 and 1631 m/sec.
In conventional ultrasound systems, the speed of sound is pre-set to be approximately 1540 m/sec. and cannot be changed. While this estimate provides fairly clear images, the image resolution can be improved by refining the speed of sound estimate. Because the tissue under examination may not be homogenous, and because the speed may vary within tissues of the same type, there is no way of preprogramming the optimum speed prior to examination of the tissue. Therefore, there is a need for an ultrasound system that can automatically estimate the speed of sound in the tissue under examination in order to increase the resolution of an ultrasound image.